1. Field of the Invention
This invention relates to a CCD solid-state image pickup device, and particularly to a CCD solid-state image pickup device in which a charge accumulation time of a photosensitive unit is adjustable.
2. Prior Art
In general, a CCD (charge-coupled device) solid-state image pickup device such as a linear sensor (one-dimensional image sensor) comprising an array of plural photosensitive units which are one-dimensionally arranged has been frequently used in an image input portion of a facsimile machine, an image scanner, a digital copying machine or the like.
A representative conventional linear sensor is shown in FIG. 4. As shown in FIG. 4, the conventional linear sensor includes a sensor array 22 comprising a number of one-dimensionally arranged photosensitive units each of which comprises a photodiode for accumulating charges in accordance with an incident light thereto, a read-out gate 23 disposed at one side of the sensor array 22 and a transfer register 24 for receiving the accumulated charges from the photosensitive units through a switching operation of the read-out gate 23. The transfer register 24 is disposed at the one side of the sensor array such that the read-out gate 23 is positionally interposed between the sensor array 22 and the transfer register 24.
The following signal output operation is carried out in the conventional CCD solid-state image pickup device thus constructed. A read-out pulse .phi.r is supplied to the read-out gate 23 during a read-out period to read out a signal charge accumulated in each photosensitive unit of the sensor array 22 and transfer it through the read-out gate 23 to the transfer register 24. Thereafter, during a transfer period, the transfer register 24 is supplied with two-phase clock pulses .phi.1 and .phi.2 having opposite phases so that each signal charge is transferred unidirectionally (in a direction as indicated by an arrow) and passed through an FDA (floating diffusion amplifier) 25 which are disposed at the rear stage of the transfer register 24, thereby converting each of the signal charges to a voltage signal one by one.
As one of video cameras having a number of sensor arrays is known a video camera which is equipped with an overflow drain and an overflow control gate at one side of each sensor array to have an electronic shutter function.
Such a video camera is disclosed, for example, in Japanese early Published (Not Examined) Patent No. P62-166661.
However, the conventional CCD solid-state image pickup device such as a linear sensor has an disadvantage that an output level thereof is varied even for the same subject (original) in accordance with luminosity change of a light source. That is, in a facsimile machine or a digital copying machine, a fluorescent lamp is usually used as a light source to read out an image on an original, but the fluorescent lamp is liable to be largely varied in luminosity with time lapse. Therefore, the output level which was obtained at an initial stage by reading out an image on an original is not obtained after a time elapses from the initial stage.
Further, when a scan speed of the linear sensor to the original is varied by fluctuated, the output level is also varied or fluctuated, the output level is also varied in accordance with variation or fluctuation of the scan speed.
In order to overcome the above disadvantages, for example, it is proposed to install an AGC (automatic gain control) circuit into the linear sensor. However, in this case, there occurs a new disadvantage that a noise component is liable to be mixed and the signal-to-noise ratio (S/N) of an image pickup signal from the linear sensor is degraded.